Suppressing CO formation by anion adsorption and Pt deposition on TiO2 in H2 production from photocatalytic reforming of methanol

Article abstract of DOI:10.1016/j.jcat.2007.10.026

H₂ with ultra-low CO concentration less than 10 ppm was produced via photocatalytic reforming of methanol on Pt/TiO₂ catalyst adsorbed with sulfate or phosphate ion. The platinum loaded on TiO₂ not only enhances the rate o

Full text of DOI:10.1016/j.jcat.2007.10.026

Journal of Catalysis 253 (2008) 225–227
www.elsevier.com/locate/jcat
Research Note
Suppressing CO formation by anion adsorption and Pt deposition on TiO2
in H2 production from photocatalytic reforming of methanol
Guopeng Wu, Tao Chen, Xu Zong, Hongjian Yan, Guijun Ma, Xiuli Wang, Qian Xu, Donge Wang,
Zhibin Lei, Can Li ∗
State Key Laboratory of Catalysis, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, Dalian, 116023, China
Received 2 September 2007; revised 21 October 2007; accepted 30 October 2007
Abstract
H
with ultra-low CO concentration less than 10 ppm was produced via photocatalytic reforming of methanol on Pt/TiO catalyst adsorbed
2
2
with sulfate or phosphate ion. The platinum loaded on TiO not only enhances the rate of H production but also considerably suppresses CO
2
2
formation. The sulfate and phosphate ions adsorbed on TiO were found to further suppress the CO formation, without evidently decreasing the
2
rate of H production.
2
© 2007 Elsevier Inc. All rights reserved.
Keywords: Photocatalysis; Methanol reforming; CO concentration; H production; Inorganic anions; Pt/TiO
2
2
1. Introduction
ode catalyst [10]. So, it is highly desired to suppress the CO
formation in photocatalytic reforming of methanol.
This communication reports that the CO concentration in H2
can be dramatically reduced to several ppm via photocatalytic
reforming of methanol on TiO2-based catalyst by depositing Pt
on TiO2 in combination with the addition of trace amount of
inorganic anions, such as SO24− and H2PO−, in the reaction sys-
tem, without evidently decreasing the rate4of H2 production.
The concerns about the depletion of fossil fuel reserves
and the pollution caused by continuously increasing energy
demands make H2 an attractive alternative energy source [1].
Methanol is one of the most promising sources for hydrogen
production because it has a high hydrogen/carbon ratio and
can also be obtained either from fossil resources or from bio-
mass [2].
H2 production from photocatalytic reaction attracts much
attention in the past few years [3–5]. Titania-based catalysts,
such as Pt/TiO2 [6,7], are efficient catalysts for photocatalytic
reforming of methanol to produce hydrogen. However, the pri-
mary H2 production from photocatalytic reforming of methanol
was always accompanied by the formation of the by-product
carbon monoxide [8,9], as shown in Scheme 1.
2. Experimental
All chemicals used in these experiments were of analytical
reagent grade and used without further treatment. The Pt/TiO2
catalysts were prepared by incipient wetness method. The com-
mercial Degussa P25 power (composed of 20 wt% rutile and
80 wt% anatase, BET surface area 53 m2/g) was impregnated
with an aqueous solution of H2PtCl6·6H2O, followed by dry-
ing in an oven at 383 K overnight. The obtained powder was
ground and then reduced at 473 K in flowing hydrogen for 2 h.
CO present in H2 is harmful to many catalysts, particu-
larly harmful to the Pt-based anode catalyst in proton exchange
membrane (PEM) fuel cells because CO even with a very low
concentration could lead to the poisoning of the Pt-based an-
*
Corresponding author. Fax: +86 411 84694447.
Scheme 1. Photocatalytic reforming of methanol for hydrogen production.
E-mail address: canli@dicp.ac.cn (C. Li).
0021-9517/$ – see front matter © 2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.jcat.2007.10.026

226
G. Wu et al. / Journal of Catalysis 253 (2008) 225–227
The photocatalytic reaction was performed with a Pyrex re-
cation, Na+, the variations in the rate of H2 production and CO
selectivity are mainly ascribed to the effect of different anions.
The order of suppression ef−fect increases in the following se-
quence: Cl− <NO3− <HCO3 < SO42− <H2PO−4 . The CO con-
centration can be decreased to about 10 ppm if a small amount
of Na2SO4 or NaH2PO4 was added in the reaction system.
For example, the CO concentration is dramatically decreased
from about 2500 ppm to 10 ppm on the addition of sulfate of
0.1 mM, or the addition of phosphate of 0.025 mM. The CO
concentration could be further decreased to even few ppm if the
concentration of sulfate or phosphate increased up to 0.3 mM.
However, as shown in Fig. 3, no evident decrease in the H2
production rate is observed after the addition of most of these
anions in the reaction solution, except for phosphate ions, which
reduces the rate of H2 production possibly by decreasing the pH
value of the reaction solution [11].
action cell connected to a closed gas circulation and evacuation
system. The photocatalyst was dispersed in a reaction cell con-
taining 200 ml methanol aqueous solution. The light source was
a 300 W Xe lamp. A shutter window and a water filter were
placed between the Xe lamp and the reaction cell in order to
remove IR light illumination. Prior to illumination, the reac-
tion system was deaerated by evacuation. The gaseous products
were analyzed by an on-line gas chromatograph (Shanghai GC-
920, TDX-01 carbon molecular sieve packed column, Ar carrier
gas). The chromatograph was equipped with a thermal conduc-
tivity detector, a flame ionization detector and a methanizer. The
amount of H2 was measured by the thermal conductivity de-
tector. The flame ionization detector was used to analyze CO,
which was converted into CH4 after passing the methanizer.
Response factors for gas products were obtained by using ap-
propriate standards.
Surface intermediate species, such as CH2O(a), CH2OO(a)
and HCOO(a) are always detected by in situ FT-IR spec-
troscopy from photocatalytic reforming of methanol on Pt/TiO2
catalyst [12]. It was proposed that photocatalytic reforming of
3. Results and discussion
Fig. 1 shows the rate of H2 production and the molar ratio of
CO/H2 in photocatalytic reforming of methanol on pure TiO2
(Degussa P25) and Pt/TiO2 catalysts. For pure TiO2, the rate
of H2 production is very low and the CO concentration in H2
is as high as 30,000 ppm. However, after loading appropriate
amount Pt on TiO2, the rate of H2 production is significantly
increased and CO concentration is found to decrease evidently.
When Pt loading is higher than 0.05 wt%, the rate of H2 pro-
duction reaches a plateau and the CO concentration in H2 drops
to a very low level, i.e. from 30,000 ppm to less than 10 ppm.
For a Pt/TiO2 catalyst with Pt loading less than 0.05 wt%,
CO concentration in H2 is still very high, e.g. about 2500 ppm
CO in H2 with 0.02 wt% Pt loading. As shown in Fig. 2, it
is interesting to find that the CO concentration (expressed as
the molar ratio of CO/H2) can be further decreased by the ad-
dition of some inorganic anions in the reaction system. The
addition of Na2SO4, NaH2PO4, NaHCO3, NaNO3, and NaCl
in the reaction solution can effectively suppress the CO for-
mation, although this suppression extent is dependent on the
choice of different anions. Since all the salts contain the same
2−
−
−
−
3
−
Fig. 2. Effect of inorganic anions (SO , H PO , HCO , NO , and Cl
)
2
4
4
3
on CO selectivity (expressed as the molar ratio of CO/H ) in photocatalytic
2
reforming of methanol on Pt/TiO catalyst. Conditions: 200 ml 5.0 M methanol
2
solution; 0.1 g 0.02 wt% Pt/TiO ; 300 W Xe lamp.
2
2− − −
−
Fig. 3. Effect of inorganic anions (SO , H PO , HCO , NO , and Cl ) on
2
4 4 3
−
Fig. 1. The rate of H production and molar ratio of CO/H as a function of
2
2
3
different amount of Pt loaded on TiO in photocatalytic reforming of methanol.
the rate of H production in photocatalytic reforming of methanol on Pt/TiO
2
2
2
Reaction conditions: 200 ml 5.0 M methanol solution; 0.3 g Pt/TiO catalyst;
catalyst. Conditions: 200 ml 5.0 M methanol solution; 0.1 g 0.02 wt% Pt/TiO ;
2
2
300 W Xe lamp.
300 W Xe lamp.

G. Wu et al. / Journal of Catalysis 253 (2008) 225–227
227
methanol proceeded stepwise involving at least two intermedi-
ates, formaldehyde and formic acid [7].
[CH3O], [CH2O], [HCOO] and finally CO2 or some CO. The
derived formic acid species adsorbed at surface defect sites
(mainly oxygen vacancy sites) could easily decompose into CO
and water. The deposited Pt or adsorbed anions suppress CO
formation by preferentially occupying the surface defect sites
on TiO2, consequently blocking the route to CO formation.
This study sheds light on the mechanism of hydrogen pro-
duction with ultra-low CO concentration from photocatalytic
reforming of not only methanol but also biomass, because the
final intermediates such as CHO− and HCOO− derived from
biomass reforming are very similar to those of methanol.
In summary, H2 with ultra-low CO concentration was pro-
duced via photocatalytic reforming of methanol on Pt/TiO2
catalyst with adsorbed SO24− or H2PO−. The formation of CO
can be greatly suppressed by deposit4ing Pt or adsorption of
small amount of inorganic anions, such as SO24− and H2PO4−
on TiO2. The deposited Pt suppresses the CO formation via
preferentially occupying the surface defect sites, while the inor-
ganic ions compete to adsorb at surface defect sites where CO
is formed mainly via the dehydration of formic acid species.
We found that both H2 production and CO formation
were considerably suppressed in photocatalytic degradation of
formic acid on Pt/TiO2 catalyst when a small amount of phos-
phate or sulfate ions were added in the initial HCOOH solution
(supporting information Fig. S1). For the pure TiO2 catalyst,
the suppression effect of the phosphate and sulfate ions on CO
formation is comparable to that for Pt/TiO2 catalyst (supporting
information Fig. S2). When SO24− or H2PO4− is pre-adsorbed on
Pt/TiO2 catalyst, CO production is greatly suppressed in photo-
catalytic reforming of CH3OH as well as HCOOH (supporting
information Fig. S3).
The inorganic anions affect the photocatalytic reaction most
likely by means of competitive adsorption on the active sites
of TiO2 catalyst [13,14]. The affinity of these anions for TiO2
surface is in the order: Cl− <NO3− <HCO−3 <SO24− <H2PO4−
[13–15], which is the same order as for the suppression ef-
fect on CO formation in photocatalytic reforming of methanol.
The competitive adsorption of HCOO− with various anions on
Pt/TiO2 was investigated by FT-IR (as shown in supporting in-
formation Fig. S4). The spectra of Pt/TiO2 with pre-adsorbed
Cl−, NO−3 and HCO3− are similar to that of Pt/TiO2, indicat-
ing that Cl−, NO3− and HCO−3 anions are weakly adsorbed on
TiO2 or even not adsorbed. However, the spectra of Pt/TiO2
with pre-adsorbed SO42− and H2PO−4 show some IR absorption
in the region 900–1300 cm−1. These FT-IR bands clearly indi-
cate that SO24− and H2PO4− were strongly adsorbed on TiO2,
most likely on the surface defect sites of TiO2 (mainly oxygen
vacancy sites on TiO2).
Acknowledgment
The support of the National Natural Science Foundation of
China (Grant Nos. 20403018, 20503034) and National Basic
Research Program of China (Grant No. 2003CB214504) are
gratefully acknowledged.
Supporting information
Supporting information for this article may be found on Sci-
enceDirect, in the online version.
Based on the above results, it is proposed that the adsorption
of formic acid on the defect sites of TiO2 may be competi-
tive with phosphate or sulfate anions. The by-product CO is
produced at the defect sites on the surface of TiO2 via the de-
hydration reaction of the intermediate formic acid species. The
CO production is suppressed by sulfate or phosphate ions be-
cause these anions may occupy the defect sites of TiO2 and
make the probability of formic acid adsorbed on TiO2 decrease.
The recombination of photogenerated electrons and holes at
the surface defect sites of TiO2 (mostly oxygen vacancy sites)
was observed by a characteristic luminescence band at about
505 nm [16]. It was found that the visible luminescence band
of TiO2 was easily quenched by the Pt deposited on the surface
of TiO2, and the luminescence intensity became weaker with in-
creasing the Pt loading (supporting information Fig. S5). Based
on the fact that the deposited Pt could decrease the intensity
of visible luminescence band and simultaneously suppresses
the CO formation, it is deduced that the surface defect sites,
which are usually the recombination sites for the photogener-
ated electrons and holes, are the active sites responsible for the
CO formation. Therefore, the deposition of Pt on the surface
defect sites can effectively suppress the CO formation.
Please visit DOI: 10.1016/j.jcat.2007.10.026.
References
[1] R.D. Cortright, R.R. Davda, J.A. Dumesic, Nature 418 (2002) 964.
[2] C.N. Hamelinck, A.P.C. Faaij, J. Power Sources 111 (2002) 1.
[3] H. Kato, K. Asakura, A. Kudo, J. Am. Chem. Soc. 125 (2003) 3082.
[4] Z. Lei, G. Ma, M. Liu, W. You, H. Yan, G. Wu, T. Takata, M. Hara,
K. Domen, C. Li, J. Catal. 237 (2006) 322.
[5] K. Maeda, K. Teramura, D. Lu, T. Takata, N. Saito, Y. Inoue, K. Domen,
Nature 440 (2006) 295.
[6] Y.Z. Yang, C.H. Chang, H. Idriss, Appl. Catal. B 67 (2006) 217.
[7] T. Kawai, T. Sakata, J. Chem. Soc. Chem. Commun. (1980) 694.
[8] Z. Zou, J. Ye, R. Abe, H. Arakawa, Catal. Lett. 68 (2000) 235.
[9] A. Galin´ska, J. Walendziewski, Energy Fuels 19 (2005) 1143.
[10] X. Cheng, Z. Shi, N. Glass, L. Zhang, J. Zhang, D. Song, Z. Liu, H. Wang,
J. Shen, J. Power Sources 165 (2007) 739.
[11] G.R. Bamwenda, S. Tsubota, T. Nakamura, M. Haruta, J. Photochem. Pho-
tobiol. A 89 (1995) 177.
[12] T. Chen, Z. Feng, G. Wu, J. Shi, G. Ma, P. Ying, C. Li, J. Phys. Chem.
C 111 (2007) 8005.
[13] Y. Li, G. Lu, S. Li, Appl. Catal. A 214 (2001) 179.
[14] M. Abdullah, G.K.C. Low, R.W. Matthews, J. Phys. Chem. 94 (1990)
6820.
[15] H.Y. Chen, O. Zahraa, M. Bouchy, J. Photochem. Photobiol. A 108 (1997)
37.
The mechanism of CO formation in photocatalytic reform-
ing of methanol on Pt/TiO2 catalyst can be proposed as fol-
lows. The photogenerated holes migrate to the surface of TiO2
and oxidize adsorbed CH3OH to form the intermediate species
[16] J. Shi, J. Chen, Z. Feng, T. Chen, Y. Lian, X. Wang, C. Li, J. Phys. Chem.
C 111 (2007) 693.